This invention relates generally to the field of optical signal transmission and in particular to the reduction of intensity noise in such systems.
Semiconductor lasers generally are an inexpensive source of coherent optical power. Semiconductor lasers can be used in optical communication systems to transmit electrical signals over optical fiber using direct modulation or external modulation. In direct modulation, an electrical input signal modulates the bias current of the laser. In external modulation, a continuous-wave (CW) laser is used and the electrical input signal modulates the transmission of light through an optical modulator. Typically the modulator is an electro-optic crystal placed in an interferometer to convert phase modulation to intensity modulation.
The performance of optical systems using semiconductor lasers is degraded by the high intensity noise produced by the semiconductor laser. This intensity noise manifests itself as electrical noise at the output of the optical system. For high dynamic range applications such as communicating with remotely-located radar antennas, analog optical systems are required to have both low noise and low distortion. Typically, in optical communication systems an expensive Nd:YAG laser is used as the laser source due to its low noise. A semiconductor diode laser is potentially the lowest-cost laser source useful for external modulation, however, it requires noise suppression to be an effective optical source for communication systems using external modulation. The intensity noise of a semiconductor laser can be reduced by passing the light through an optical filter which has the appropriate variation in transmission as a function of wavelength.
In a semiconductor laser or semiconductor amplifier, fluctuations of the carrier density level that determines the material gain is the source of intensity noise. Changes in the carrier density change both the gain and refractive index of the gain medium. The ratio between the real part of the refractive index to the imaginary part of the refractive index (which is proportional to the gain) is defined as the linewidth enhancement factor a. When the laser is modulated, a large linewidth enhancement factor a produces a large frequency modulation (FM) signal corresponding to the intensity modulation. Because of this correlation between gain and phase changes in the gain medium, carrier fluctuations also produce optical FM noise that is correlated with the intensity noise of the laser. Thus, if the optical signal is passed through an optical filter having the appropriate transfer function slope, the FM noise of the optical carrier generates correlated intensity variations at the output of the optical filter. These correlated intensity variations can be out of phase with the original intensity noise, leading to substantially lower total intensity noise.
An optical filter noise reduction technique has generally not been applied to semiconductor laser optical communication systems because the method as previously disclosed has not been suitable in such systems using direct modulation or external modulation links. This is because if the optical filter noise reduction technique were applied to a direct modulation link, the signal to noise ratio at the link output would be reduced because the modulation signal would be reduced by at least as much as the intensity noise. If this optical filter noise reduction technique were applied to an external modulation communication system, the optical insertion loss of the noise reduction filter can lead to unacceptable degradation of link performance. Therefore, what is needed is an external modulation configuration with an optical filter that has the appropriate transmission slope for noise reduction, but does not introduce optical insertion loss.
The invention relates to a method and apparatus for modulating the intensity and improving the noise figure of an optical source in an optical communication system. An optical filter is used to reduce phase correlated intensity noise in the system without adding optical insertion loss. The filter is included in an interferometer having unbalanced (i.e., unequal) path lengths and an internal phase modulator. The method and apparatus are suitable for laser sources in which the refractive index of the laser medium varies with a change in gain. For example, links using semiconductor diode laser sources can benefit from this method.
The apparatus includes an optical source and an interferometer. The interferometer includes an input port, an output port, a first optical path and a second optical path. The first and second optical paths are each in optical communication with the input port and the output port. The length of the second optical path differs from the length of the first optical path by a predetermined optical path length difference. The apparatus also includes a modulator having a modulator electrical input. The modulator is located in the second optical path of the interferometer. The modulator generates an optical phase delay in response to a signal received at the modulator electrical input. The interferometer provides an optical signal having reduced phase correlated intensity noise in response to the predetermined optical path length difference.
In one embodiment the optical source is a semiconductor laser diode. In another embodiment the apparatus includes a detector in optical communication with the interferometer output port. In another embodiment the predetermined optical path length difference is adjustable.
In one embodiment the apparatus includes a reflective element in which receives radiation transmitted by the first and second optical paths through the output port. The received radiation is reflected back into the output port and transmitted back through the first and second optical paths.
The method includes the steps of providing a source of optical radiation having intensity noise, splitting the optical radiation from the source into a first optical signal and a second optical signal, and delaying the optical phase of the second optical signal relative to the first optical signal by a predetermined phase delay. The method also includes the steps of modulating the phase of the first or second optical signal in response to an electrical signal and combining the first and second optical signals to generate an output optical signal having a reduced intensity noise. In one embodiment the predetermined phase delay produces a periodic variation in interferometer output intensity as a function of wavelength that is an integer multiple of the mode spacing of the optical source.